It has been known for some time that the surface composition of a metal may have a significant impact on its utility. It has been known to treat steel to produce an iron oxide layer that is easily removed. It has also been known to treat steel to enhance its wear resistance. As far as Applicants are aware there is not a significant amount of art on selecting a steel composition to support an overcoat (preferably on chromia) to significantly reduce coking in hydrocarbon processing.
It is known that some steels (e.g. high chromium steels) will produce a chromia coating under certain conditions. It is predicted that chromia stability against coking is significantly reduced under conditions where the carbon activity is about 1 (e.g. with a deposit of a carbon or coke layer). For example at temperatures greater than about 950° C. and at low oxygen partial pressures chromia starts to be converted to chromium carbides. Such carbide formation leading to volume expansion, embrittlement and possible spallation, thereby leaving the surface unprotected and reducing the coking resistance of the steel tubes. The present invention seeks to address this problem.
U.S. Pat. No. 3,864,093 issued Feb. 4, 1975 to Wolfla (assigned to Union Carbide Corporation) teaches applying a coating of various metal oxides to a steel substrate. The oxides are incorporated into a matrix comprising at least 40 weight % of a metal selected from the group consisting of iron, cobalt, and nickel and from 10 to 40 weight % of aluminum, silicon and chromium. The balance of the matrix is one or more conventional metals used to impart mechanical strength and/or corrosion resistance. The oxides may be oxides or spinels. The patent teaches that the oxides should not be present in the matrix in a volume fraction greater than about 50%, otherwise the surface has insufficient ductility, impact resistance, and resistance to thermal fatigue. The reference does not teach overcoatings to protect chromia nor does it suggest the composition of a steel adapted to support such a coating.
U.S. Pat. No. 5,536,338 issued Jul. 16, 1996 to Metivier et al. (assigned to Ascometal S.A.) teaches annealing carbon steels rich in chromium and manganese in an oxygen rich environment. The treatment results in a surface scale layer of iron oxides slightly enriched in chromium. This layer can easily be removed by pickling. Interestingly, there is a third sub-scale layer produced which is composed of spinels of Fe, Cr and Mn. This is opposite to the subject matter of the present patent application. U.S. Pat. No. 4,078,949 issued Mar. 14, 1978 to Boggs et al. (assigned to U.S. Steel) is similar to U.S. Pat. No. 5,536,338 in that the final surface sought to be produced is an iron based spinel. This surface is easily subject to pickling and removing of slivers, scabs and other surface defects. Again this art teaches away from the subject matter of the present invention.
U.S. Pat. No. 5,630,887 issued May 20, 1997 to Benum et al. (assigned to Novacor Chemicals Ltd. (now NOVA Chemicals Corporation)) teaches the treatment of stainless steel to produce a surface coating having a thickness from about 20 to 45 microns, comprising from 15 to 25 weight % of manganese and from about 60 to 75 weight % of chromium.
The reference is silent about the composition of the outer layer and the presence of a chromia layer.